When a transmitter and a receiver communicate with each other by radio communication and a radio wave scatterer that reflects and/or diffracts electromagnetic waves is present between the transmitter and the receiver, a plurality of radio propagation paths are formed between the transmitter and the receiver. When the plurality of radio propagation paths is simply used together, the radio communication is interrupted when any one of the propagation paths is altered by being affected naturally or artificially. For this reason, it is desirable to eliminate the correlation of the plurality of radio propagation paths.
For example, Patent Literature 1 listed below states that “According to the present invention, in connection with a demodulation system of MIMO-OFDM transmission, when performing MIMO-OFDM transmission using multiple transmission antennas and reception antennas in an outdoor line-of-sight environment by assigning different polarizations such as orthogonal polarizations to respective transmission antennas and reception antennas, using a cross polarization power ratio measured on a receiver in adjusting XPD of transmission antennas of the transmitter and the receiver efficiently reduces correlation of propagation paths and inhibits the reduction of the effect of receive diversity, which is due to the use of polarized waves, to improve the MIMO-OFDM transmission characteristics” (see paragraph 0022).